11.05.11

It’s been almost ten years since I’ve actively used the Java programming language. In the mean time, I’ve mostly used C++. I’ve had to pick up a bit of Java again recently. Here are a few of the things that I found surprising or notable. These are all variants on “that’s changed in the last ten years” or “that’s not how C++ does it.”

The Java compiler enforces what would be conventions in C++.
For example, “public class Foo” has to be in Foo.java. In C++, this would just be a convention. You can use “private class” when you’re playing around with test code and want to use only a single file. Similarly, class foo.Bar needs to be in “foo/Bar.java”.

Java Packages are a more pervasive concept than namespaces in C++.
There’s a “default package”, but using this prevents you from loading classes by name: Class.fromName(“Foo”) won’t work, but Class.fromName(“package.Foo”) will. Classes in your current package are auto-imported, which surprised me at first. The default visibility for methods/fields in Java is “package private”, which has no analogue in C++.

Java keeps much more type information at runtime time than C++ does.
The reflection features (Class.getMethods(), Method.getParameters(), etc.) have no equivalent in C++. This leads to some seemingly-magical behaviors, e.g. naming a method “foo” in a Servlet can cause it to be served at “/foo” without you saying anything else. Not all information is kept though: you can get a list of all packages, but not a list of all classes in a package. You can request a class by its name, but you can’t get a list of all classes. You can get a list of all the method names in a class, but you can’t get a list of all the parameter names in a method.

Java enums are far richer than C/C++ enums.
enums in Java are more like classes: they can have constructors, methods, fields, even per-value method implementations. I really like this. Examples:

Java is OK with a two-tier type system.
At its core, C++ is an attempt to put user-defined types on an equal footing with built-in types like int and char. This is in no way a goal of Java, which is quite content to have a two-tier system of primitive and non-primitive types. This means that you can’t do Map<int, int>, for instance. You have to do Map<Integer, Integer>. Autoboxing makes this less painful, but it’s still a wart in the language that you have to be aware of.

One concrete example of this is the “array[index]” notation. In C++, this is also used for maps. There’s no way to do this in Java, and I really miss it. Compare:

map[key] += 1;

to

map.put(key, 1 + map.get(key));

which has more boilerplate and is more error-prone, since you might accidentally do:

map.put(key, 1 + other_map.get(key));

The designers of Java Generics learned from the chaos of C++ templates.
Generic classes in Java are always templated on types: no more insane error messages. You can even say what interface the type has to implement. And there’s no equivalent of method specialization, a C++ feature which is often misused.

Variables/fields in Java behave more like C++ pointers than C++ values.
This is a particular gotcha for a field. For example, in C++:

Java constructors always require a trailing (), even if they take no parameters.
This is a minor gotcha, but one I find myself running into frequently. It’s “new Foo()” instead of “new Foo” (which is acceptable in C++).